Windows 10 and later: birdfont-3.20.4-free.exe 64 bit OS
Windows 7 and later: birdfont-3.20.3-free.exe
Windows Vista and later: birdfont-2.13.0.exe
Mac OS 10.12 and later: birdfont-3.20.2-free.dmg
Mac OS 10.11 and later: birdfont-2.19.4.dmg
Mac OS 10.6 and later: birdfont-2.17.3.dmg
You need to allow Mac OS to run applications from uncertified developers on old versions of Mac OS.
Linux x86_64: birdfont-3.20.2-free.flatpak
The latest stable release for mac is birdfont-3.20.2-free.dmg. It works with 10.12 and later versions of Mac OS. Download birdfont-2.19.4.dmg if you have Mac OS 10.11 or later. Download birdfont-2.17.3.dmg if you have an even older version of Mac OS (10.6 or later). You need to allow Mac OS to run applications from uncertified developers on older version of Mac OS. Right click on the app and choose open or open Apple menu > System Preferences… > Security & Privacy > General and select Allow applications from anywhere.
flatpak remote-add --from gnome https://sdk.gnome.org/gnome.flatpakrepo
flatpak install gnome org.gnome.Platform/x86_64/3.22
flatpak install birdfont-3.20.2-free.flatpak
Run this if you want to check the signature:
flatpak install --gpg-file=birdfont.asc birdfont-3.20.2-free.flatpak
A package is available for OpenBSD in graphics/birdfont.